Initial success utilizing O6-benzylguanine (BG) to inhibit O6-alkylguanine DNA alkyltransferase (AGT) provided proof in principal that AGT plays a major role in BCNU resistance in human tumors and that BG inactivation can markedly potentiate the effect of BCNU against these tumors in xenograft models. The hypothesis to be tested is that optimizing AGT inactivation will significantly improve the therapeutic response of agents which form cytotoxic O6-alkylguanine DNA adducts. First, simple inhibition of AGT is insufficient to maximize efficacy: most AGT+ tumors remain more resistant to BCNU than AGT-tumors. Superinhibitory doses of BG will be used with DNA methylation to maximize BCNU cytotoxicity. Secondly, a breast cancer cell line with acquired resistance to BG/BCNU has recently been identified, suggesting that resistance mechanisms will limit the efficacy of BG. The investigators will identify these mechanisms and develop strategies to overcome them. Third, tumors vary greatly in AGT and in the level of MGMT mRNA which affects the rate of AGT synthesis. How these influence the level of BCNU resistance has not been evaluated in vivo. Colon and breast tumor xenograft tumor models which mimic this diversity through either high AGT expression or BG resistance will be used as models of in vivo tumors which may be difficult to treat with BG/BCNU. These treatment refractory tumors will be exposed to newer BG-derivatives and with modified dose strategies to maximize biochemical modulation of resistance. Finally, BG/BCNU is toxic to normal tissues, particularly the marrow. A recently identified mutant AGT completely resistant to BG will be studied using gene transfer into marrow progenitors to provide selective marrow protection against combinations of BG for BG derivatives] & BCNU. Selective marrow protection may enhance potential clinical applications of AGT depletion. These studies are designed to enhance the utility of BG and derivative compounds as AGT inhibitors to overcome tumor resistance to this class of alkylating agents.